This invention relates to a microfabricated pressure transducer formed on a multilayer substrate.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. One example of a MEMS device is a microfabricated cantilevered beam, which may be used to switch electrical signals. Because of its small size and fragile structure, the movable cantilever may be enclosed in a cavity to protect it and to allow its operation in an evacuated environment. Therefore, upon fabrication of the moveable structure on a device wafer, the device device wafer may be mated with a lid wafer, in which depressions have been formed to allow clearance for the structure and its movement. To maintain the vacuum over the lifetime of the device, a getter material may also be enclosed in the device cavity upon sealing the lid wafer against the device wafer.
Movable MEMS devices may include actuators, sensors and switches. Microfabricated pressure sensors comprise a small but useful subset MEMS devices. Microfabricated sensors can be very sensitive to pressure changes, making them ideal for applications in which bulky machined sensors are not able to perform, or are too large, or consume too much power. Typical applications of integrated pressure sensors include microphones, biomedical instrumentation (e.g., blood and fluid pressure), vacuum sensing, wind-tunnel model instrumentation, automobile power and acceleration measurement, and even household electronics.
Generally, mechanical sensors are based on material changes caused by stress placed on a membrane or other flexible element. The most common and inexpensive type, is based on the piezoresistive effect, wherein the resistance of the piezoresistive element changes as a function of strain. Piezoresitors can be made of doped silicon or polysilicon. Polysilicon has better stability, avoids time- and temperature-variant p-n junctions, and can be used in operating temperatures up to 200° C. On the submillimeter scale of integrated devices, materials like silicon show very little or no fatigue, which is apparently a macroscale phenomenon. Thus integrated sensors can be flexed indefinitely, and have a long lifetime.
In addition to sensors based on the piezoresistive effect, there also exist high-precision sensors based on capacitive effect. A membrane is also used, with one plate of a capacitor mounted on the membrane and the other plate suspended above it, usually fabricated on a relatively inflexible material such as Pyrex glass. The deflection of the membrane changes the distance between the plates and thus changes the capacitance. Capacitors tend to be much less temperature and time variant than piezoresistors. Output of capacitive sensors is highly appropriate for switched-cap circuit design.
However, each of the aforementioned devices may be rather complex and expensive to manufacture. Accordingly, microfabricated, low cost pressure transducers have posed an unresolved problem.